A long list of polymers have been studied in the past for their utility in the formation of membranes for a variety of purposes. Of importance to the present invention is the use of such polymer membranes for separations and selective transport to modify a feed material and/or to recover one or more target species from the feed material. A host of polymers have been determined to be useful for such applications, with each material exhibiting its own benefits and drawbacks for particular membrane separation and transport applications.
An application of particular interest to the Applicant is the degassing of liquids through contact with a gas-permeable, liquid-impermeable membrane. Such liquid-gas contactors typically rely upon Henry's Law of partial pressures and Ficke's law of diffusion to drive gas transport through the membrane, while small pore size, or the absence of through-pores in a “nonporous” media, restricts or prevents liquid transport through the membrane. The development of fluoropolymers has greatly aided the membrane liquid degassing field by providing membrane polymers that are generally inert, and can be formed into a gas-permeable, liquid-impermeable membrane structure. A particular fluoropolymer of note is a class of amorphous perfluoropolymers, such as those available from Du Pont under the trade name “Teflon®”, as well as other amorphous fluoropolymers available from Asahi Glass Corporation and Solvay Solexis. Such materials are oftentimes employed in gas separation membranes for their inertness and high permeability characteristics. Membranes are typically selected for a combination of their compatibility with the contacting materials, their permeability to the targeted transport species, and their selectivity of one molecule over another. It has been shown that, while membrane selectivity may be constant as a function of the membrane thickness, the throughput (permeance) changes inversely to the thickness of the membrane. As a result, a thinner membrane is typically desired, but is limited by the decreased strength and durability as membrane thickness is reduced. It is therefore an ongoing challenge to obtain selective membranes that have the highest possible permeance without being unduly fragile. Such membranes should also be resistant to fouling, degradation, or other performance deterioration.
Membrane engineers have attempted to employ fragile membranes with desired performance properties by supporting the selective membranes with a support structure. A variety of reinforcing support structures have been previously implemented, but are typically difficult to handle, expensive, and/or degrade the performance of the primary selective membrane. Suitable structural reinforcements to thin film membranes that avoid these drawbacks have yet to be defined.
Reinforcement materials for thin film membranes have typically been in the form of lattice structures, support films, and particulate dopants. One material that has been extensively studied for its strengthening properties, though not in thin film separation membranes, is carbon nanotubes, which are recognized as a high-strength material, deriving their strength from its native sp2 bond structure. The electron cloud associated with the sp2 bonding structure functions as an interaction between proximate carbon nanotubes, such that nanotubes may be formed into coherent sheets, tapes, ribbons, ropes, and other macrofabrics, with a tensile strength that is sufficient to facilitate handling.
Internanotube forces have been noted so long as the nanotubes are well associated wherein the surfaces of proximate nanotubes can interact. The particular method of forming nanotubes into such sheets, tapes, and the like, however, can greatly affect the strength of the so-formed macro-scale nanotube structures. Carbon nanotube arrays in a sheet form are commonly known as “buckypaper”, referring to the mechanical structure of carbon nanotubes as described by Buckminster Fuller. Generally, the bonding interactions among the nanotubes are insufficient to form a buckypaper that has commercial use on its own. However, carbon nanotubes have been described as a dopant to various materials, including polymers, by mixing carbon nanotube powder into the polymer. Typically, researchers seek improved strength and/or electrical conductivity when doping polymers with carbon nanotubes.
Strength reinforcement materials, including glass fibers, carbon fibers, metal fibers, carbon nanotubes, and the like, when conventionally added as a dopant material, are dependent upon surface energy compatibility between the dopant material and the matrix for the degree of strength enhancement. Matching of the respective surface energies permits van der Walls interactions to assist in the load transfer between the dopant and the matrix. In some cases, surface energy matching is not possible without chemical modification of the reinforcing material, which chemical modification can be expensive or even impossible.
Recently, a reverse approach has been attempted, wherein a buckypaper is infused with a polymer to maintain the native strength of the carbon nanotube sheet derived from the van der Walls interactions among the nanotubes. An example of such an approach is described in U.S. Pat. No. 7,993,620, herein incorporated by reference. The non-woven carbon nanotube fabric described in U.S. Pat. No. 7,993,620 may be incorporated into composite structures by impregnating the non-woven fabric with a matrix precursor, and allowing the matrix to polymerize or thermally cure. Such composites have been described for use in impact-resistant applications, such as sporting goods protection devices, including helmets. Other carbon nanotube fabric composites are described in U.S. Patent Application Publication No. 2010/0324565, also incorporated herein by reference.
The example carbon nanotube fabric structure described above for the formation of composites is described in detail in U.S. Patent Application Publication Nos. 2009/0215344, and 2011/0316183, while an apparatus useful to synthesize nanotubes of such carbon nanotube fabrics is described in U.S. Patent Application Publication No. 2009/0117025, each of which are incorporated herein by reference.
Though composites of carbon nanotube fabrics and infused polymers have been demonstrated, the Applicant is unaware of such composites prepared as a thin film membrane for, as an example, separations. One explanation for the lack of work in this area may be due to the expectation that carbon nanotube fabrics would act similarly to other reinforcement structures that interfere with the overall permeability of the composite structure. It is well know that solid portions of conventional thin film support structures often reduce permeance performance as compared to the neat thin film separation membrane.
It is therefore an object of the present invention to provide a composite structure that exhibits desired tensile strength with a substantially reduced effective polymer film thickness.
It is another object of the present invention to provide a composite membrane incorporating a nanotube reinforcement structure that does not significantly degrade permeation performance of the separation polymer matrix.